Field of the Invention
The invention relates in general to suppressing electromagnetic interference (EMI) of double data rate synchronous dynamic random access memory (DDR SDRAM) signals, and more particularly to a circuit structure for suppressing EMI of DDR SDRAM signals.
Description of the Related Art
Wires and lines routed on a circuit board are frequently one main factor causing electromagnetic interference (EMI). When EMI is caused by signal transmission of a memory interface unit (MIU) of a double data rate synchronous dynamic random access memory (DDR SDRAM), the performance of the DDR SDRAM is often degraded, e.g., reduced data transmission rate or increased data error rate. There are many signal limes in a partial circuit diagram of an MIU of a conventional DDR SDRAM. In an embodiment, a signal line is surrounded by ground lines at the two sides. In another embodiment, two signal lines are surrounded by two ground lines at the two sides. A signal line may be used for transmit a command signal, an address signal, a data signal DQ or a data strobe signal DQS for operating the DDR SDRAM. EMI is associated with the operating frequency of the DDR SDRAM and the length of a routing line. As the operating frequency of the DDR SDRAM gets higher, e.g., 200 MHz, a predetermined length of the routing line may cause noticeably high EMI under this frequency. Taking data actually measured for example, as the operating frequency of the DDR SDRAM is 200 MHz, the EMI may reach as high as 44 dB. Such high EMI exceeds the specification of DDR SDRAM, and likely results in data transmission errors or system instability. There are several conventional methods for reducing EMI.
In one of the methods, a capacitor is coupled between a signal line and a ground area. The main purpose of coupling this capacitor is to increase the rising time of the signal, such that the energy of EMI may be reduced. However, the tradeoff is that, the coupled capacitor reduces the quality of the signal, hence degrading the overall performance of the circuit. In another method, based on the principle of metal shielding effect, a metal shielding box or a metal film, e.g., copper foil, is covered on the circuit board. However, whether the metal shielding box or metal film is applied for covering, costs are increased.